1. Field of the Invention
The invention relates to a power amplifier for delivering large currents as well as high voltages.
2. Description of Related Art
Amplifiers of this kind can be used for a large variety of applications. Even though the invention will be described on the basis of an application in the magnetic resonance imaging (MRI) technique, notably for driving the gradient coils of an MRI apparatus, the application of the invention is by no means restricted to such a technical field.
MRI systems utilize power amplifiers for driving the coils which generate so-called gradient fields. Such gradient fields are magnetic fields having a strength which varies linearly in a given co-ordinate direction in order to define the location of the image to be formed by means of the MRI apparatus by addition of this gradient field to a strong steady, uniform field. To this end, current pulses of an intensity of the order of magnitude of more than 600 A at a voltage of the order of magnitude of more than 500 V are applied to the gradient coils, the rise time of the pulses being of the order of magnitude of 0.2 ms whereas the pulse duration is of the order of magnitude of from 1 ms to 10 ms.
Thus, in the context of the present invention large currents are to be understood to mean currents of the order of magnitude of several hundreds of amperes, and high voltages are to be understood to mean voltages of the order of magnitude of one thousand volts or more.
Nowadays there is a tendency towards shorter rise times with larger maximum currents so as to reduce the time required for the acquisition of MRI information for the formation of an MRI image; this offers advantages inter alia in respect of image sharpness and also in respect of imaging of moving objects. Because the gradient coils exhibit an inductive behavior to the driving amplifier, a higher voltage is required so as to achieve a shorter rise time of the pulses. Increasing the currents and the voltages to be supplied by the amplifier, in combination with a shorter rise time, gives rise to problems concerning the electronic components in the amplifier. Losses in the semiconductor components cause a significant development of heat, giving rise to cooling problems. These problems can be mitigated partly by using a switched inverter, that is to say an amplifier whose transistors which carry the output current are switched to be either completely turned on or completely turned off. Switching to the turned-off or turned-on state is controlled by means of a pulse width modulated (PWM) signal. The output voltage of the inverter is then determined by the duty cycle of the PMW signal.
Other problems are encountered when the desired output voltage is increased and/or the desired rise time is reduced: the desired voltage may be so high that no transistors are available or, should they be available, they have a stray capacitance which is so high that the PWM switching frequency (of the order of magnitude of 20 kHz) required for the relevant application can no longer be reached. Moreover, in the case of a high supply voltage the voltage transients at the output of the amplifier, but preceding the output low-pass filter, in response to the switching of the amplifier transistors would become so high that the higher harmonic content of the output signal would become so high that the output low-pass filter would have to satisfy very severe requirements. The latter problems can be partly mitigated by using a switched amplifier of the multilevel type (multilevel inverter).
In an inverter of this type the total voltage is distributed between two or more series-connected transistors, means being provided so as to prevent the total voltage from still being present across a single transistor due to, for example inequalities of the transistors (for example, due to individual spreads caused by manufacturing tolerances). Such means consist of one or more capacitors which are connected parallel to a part of the series connection of the transistors and always carry a more or less constant fraction of the total voltage present across the series connection of the transistors. Inverters of this kind are known per se, for example from an article in EPE Journal, Vol. 3, No. 2, June 1993, pp. 99-106, entitled "Imbricated cells Multi-Level Voltage Source Inverters for High Voltage Applications".
In these known multilevel inverters use can thus be made of transistors (generally semiconductor elements), each of which has a breakdown voltage which is lower than the total voltage present across the circuit, and hence also lower than the output voltage of the circuit. In this known circuit, however, the problem in respect of the power losses in the semiconductor components and the associated cooling problems are not solved completely, notably not in respect of the required high PWM switching frequencies. These power losses consist of switching losses and conduction losses due to the resistance of the material of the various components.
Due to the high output voltages and the high switching frequency, the switching losses usually are predominant in amplifiers for high powers and high PWM switching frequencies. Such switching losses occur because the switching transistor switches over from the turned-on to the turned-off state or vice versa. In the turned-on state the current through the transistor has a given value, but the voltage across the transistor is substantially zero or very small in any case (for example, 0.5 V); in the turned-off state the voltage across the transistor has a given value, but the current across the transistor is practically zero. During the switching-over from one state to the other, however, a product of current and voltage occurs; this implies power dissipation.
It is known per se to reduce switching losses by means of a so-called "soft switching" technique. When this technique is used, the instant of the transition from the turned-on state to the turned-off state or vice versa (the switching) is chosen in such a manner that either the current through the switch is zero or practically zero ("Zero Current Switching") or the voltage across the switch is zero or practically zero ("Zero Voltage Switching"). Thus, the product of current and voltage is substantially zero in both cases. Inverters operating with such a switching mode are known per se, for example from an article in "Conference Record of the IEEE Industry Application Society Annual Meeting", 1990, Vol. 2, pp. 1228-1235, entitled "The Auxiliary Resonant Commutated Pole Converter". The inverters disclosed in the latter article, however, are not of the multilevel type, so that the described problems in respect of the high voltages are not solved.